Callose is a plant polysaccharide composed of glucose residues linked together through β-1,3-linkages, and is termed a β-glucan (β-1,3-glucan). It is thought to be manufactured at the cell wall by callose synthases (Chen and Kim, Plant Signaling & Behavior 2009, 4(6), 489-492). It is laid down e.g. at plasmodesmata, at the cell plate during cytokinesis and during pollen development. Callose plays many important roles in plants and is produced in response to e.g. wounding, infection by pathogens, aluminium and abscisic acid. Furthermore, callose deposition has been shown to affect cell-to-cell signalling, and impaired trafficking between cells has been shown to result from increased callose accumulation at the plasmodesmata (Sivaguru et al., Plant Physiology 2000, 124(3), 991-1006; Simpson et al., Plant Cell 2009, 21, 581-594). Callose polymer has chemical and physical properties distinct from the related cellulose polymer, a β-1,4-glucan (Maltby et al., Plant Physiology 197, 1158-1164).
Currently, however, callose biosynthesis is poorly characterized, and efficient tools for enhancing callose biosynthesis and accumulation have not been available. WO2010/087805 has previously described the overexpression of wild-type callose synthase. The overexpression of wild-type callose synthases appears, however, to result only in limited increases in callose biosynthesis and/or accumulation.